Process for reduction of gaseous sulfur compounds

ABSTRACT

A process for the reduction of gaseous sulfur compounds in gaseous streams. The gaseous stream is contacted with a sorber, e.g., zinc oxide, which is capable of sorbing the sulfur compounds under sulfur sorbing conditions. The sorber is present in the form of one or more layers on the surface of a monolith carrier, e.g., cordierite. The layers of the sorber have a total thickness of at least 3 g/in 3  of the carrier. The process is especially useful for the removal of gaseous sulfur compounds such as H 2 S from gaseous streams.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a process for the reduction of the levelof gaseous sulfur compounds, e.g., H₂S, in a gaseous stream bycontacting the stream with a sorber capable of absorbing such compoundsunder sulfur sorbing conditions.

[0003] 2. Discussion of the Prior Art

[0004] In many applications, it is well known that it is desirable toreduce the level of gaseous sulfur compounds such as H₂S, COS,mercaptans, etc. Many applications, e.g., fuel cells, require that thegaseous sulfur compounds in a raw fuel stream (e.g., naphtha, LPG, towngas, etc.) be reduced to as low a level as practicable in order to avoidpoisoning the environment or catalysts such as steam reformingcatalysts, water-gas shift catalysts, etc. Furthermore, fuel cellelectrodes will rapidly become inactivated as the result of high levelsof gaseous sulfur compounds in the fuel stream since the electrodesinvariably contain precious metal components, e.g., platinum, which areextremely sensitive to the presence of sulfur compounds.

[0005] There are many prior art processes involving hydrogenationdesulfuirization in which the sulfur compounds in the raw fuel streamare decomposed by hydrogenolysis at temperatures of e.g., 350 to 400° C.in the presence of e.g., Ni—Mo or Co—Mo catalysts and thereafter theresultant H₂S is then absorbed on abed of ZnO at temperatures of e.g.,300 to 400° C. However, in these processes, the level of the H₂S in thetreated stream is still too high, eg., 100 ppm and higher. However, itis well known that if the gas stream contains gaseous sulfur compoundsin as little a level as 0.25 to 25 ppm, about 90% of the surface of asteam reforming catalyst such as Ru or Ni will be covered with thesulfur compounds, thereby resulting in a rapid deterioration of thecatalyst. Furthermore, the prior art processes are typically not capableof reducing the level of gaseous sulfur compounds to very low levels,e.g., 100 ppb and lower since prior art sorbers are used underconditions wherein severe pressure drops would otherwise occur if theflow rate of the raw file gaseous stream is significantly increased inorder to improve the sorbing reaction rate.

[0006] Therefore, there is a need for a process which will “polish” adesulirized fuel stream containing on the order of 25 ppm gaseous surfurcompounds such as H₂S and further reduce the level of such compounds inan efficient manner to a level of less than 100 ppb for a period of atleast one hour, i.e. before “breakthrough” commences. For the purposesof this invention, “breakthrough” shall be understood to mean that thelevel of the gaseous sulfur compounds commences rising above 100 ppb.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The invention relates to an improved process for the reduction ofgaseous sulfur compounds, e.g., H₂S present in a gaseous stream,especially in a gaseous stream which has been pre-treated to reduce thelevel of gaseous sulfur compounds below 25 ppm. The improvement residesin contacting the stream with a sorber capable of absorbing suchcompounds under sulfur sorbing conditions, with the sorber being presentin the form of one or more layers on the source of a monolith carrier.

[0008] It was found that in the first 30 minutes of use at 400° C., bedof zinc oxide pellets allowed 20% of an 8 ppm H₂S steam to breakthrough. In contradistinction, when a monolith carrier containing 15% byweight of the zinc oxide content of the pellet bed, less than 5% of theoriginal concentration of the H₂S was allowed to pass through Inparticular, it was found that highly dispersed zinc oxide present asmultiple layers on a monolith carrier such as cordierite reduces theconcentration of hydrogen sulfide in a gas stream to a much greaterextent than almost 10 times the amount of zinc oxide in pellet form in abed. The monolith coated with zinc oxide represent a device with afraction of the back pressure of abed of finely divided zinc oxide.Although the hydrogen sulfide capacity of the zinc oxide-coated monolithcarrier is a fraction of that of the zinc oxide pellets, the former iscapable of reducing the hydrogen sulfide concentration by ≧95% ascompared to a reduction in hydrogen sulfide of 70-90% using zinc oxidepellets.

[0009] The layers of the sorber on the monolith carrier are such thatthe layers will have a total thickness of at least about 3 g/in³ of thecarrier, preferably at least 3.5 g/in³ of the carrier. Preferably, thesorber will be present in the form of at least three layers on thesurface of the monolith carrier. It is also preferred that the sorber bepresent on the surface of the monolith carrier in the form of particleshaving an average particle size of 90%<10μ.

[0010] Typically, the sorber will comprise one or more metal compoundswherein the metal is selected from the group consisting of zinc,calcium, nickel, iron, copper and mixtures thereof. The preferred metalis zinc and the preferred metal compounds are zinc oxide and zinctitanate.

[0011] The process of the present invention operates most efficientlywhen the gaseous sulfur compound in the gaseous stream prior to contactwith the sorber is primarily H₂S present in an initial concentration ofabout 0.15 to about 25 ppm, preferably 0.25 to 10 ppm, and the stream ispassed into contact with the sober at a volumetric hourly rate of about500 to about 100,000 volumes, preferably 2,500 to 15,000 volumes, pervolume of monolith carrier.

[0012] Typically, the process of the present invention will result in areduction of the H₂S from its initial concentration to a level of lessthan 100 ppb for a period of at least one hour, preferably less than 50ppb for at least 4 hours.

[0013] The sorber is disposed on the surface of a monolith carrier,preferably of the type comprising one or more monolithic bodies having aplurality of finely divided gas flow passages extending therethroughSuch monolith carriers are often referred to as “honeycomb” typecarriers and are well known in the prior art. A preferred form of thecarrier is made of a refractory, substantially inert, rigid materialwhich is capable of maintaining its shape and a sufficient degree ofmechanical conditions at high temperatures of about 1450° C. Typically,a material is selected for use as the carrier which exhibits a lowthermal coefficient of expansion, good thermal shock resistance andpreferably low thermal conductivity.

[0014] Two general types of materials of construction for monolithcarriers are known. One is a ceramic-like porous material composed ofone or more metal-oxides, e.g., alumina, alumina-silica,alumina-silica-titania, mullite, cordierite, zirconia, zirconia-ceria,zirconia-spinel, zirconia-mullite, siliconcbide, etc. A particularlypreferred and commercially available material for use as the carrier forthe present invention is cordierite, which is an alumina-magnesia-silicamaterial.

[0015] Monolith carriers are commercially available in various sizes andconfigurations. Typically, the monolithic carrier would comprise, e.g.,a cordierite member of generally cylindrical configuration (either roundor oval in cross section) and having a plurality of parallel gas flowpassages of regular polygonal cross sectional extending therethrough.The gas flow passages are typically sized to provide from about 50 toabout 1,200, preferably 200-600, gas flow channels per square inch offace area The second major type of preferred material of constructionfor the monolith carrier is a heat- and oxidation-resistant meal, suchas stainless steel or an iron-chromium alloy. Monolith carriers aretypically fabricated from such materials by placing a flat and acorrugated metal sheet one over the other and rolling the stacked sheetsinto a tubular configuration about an axis parallel to theconfigurations, to provide a cylindrical-shaped body having a pluralityof fine, parallel gas flow passages, which may range, typically, fromabout 200 to about 1,200 per square inch of face area.

[0016] The monolith carrier may also be present in the form of a ceramicor metal foam. Monolith carriers in the form of foams are well known inthe prior art, e.g., see U.S. Pat. No. 3,111,396 and SAE Technical Paper971032, entitled “A New Catalyst Support Structure For AutomotiveCatalytic Converters” (February, 1997).

[0017] The following procedure may be used to prepare the coatedmonolith carrier employed in the process of the invention:

[0018] Commercially available zinc oxide, e.g., “Halder-Topsoc 1TZ-5”extrudates, is ball milled for about 12 hours using alumina balls andsufficient water to prepare a suspension of 30 wt. % solids. Thereafter,the particle size distribution is measured. If 90% of the particles are<10μ the milling is complete; otherwise the milling is continued untilsuch particle sized distribution has been achieved

[0019] Thereafter, the slurry is placed in a container of sufficientdepth such that a monolith carrier can be fully immersed in the slurry.For example, a monolith carrier of 1.5 inches in depth will require aslurry depth of about 2 inches. The monolith carrier is dipped in thezinc oxide slurry and the free-flowing excess is allowed to drain off.Blocking of the channels of the monolith carrier is minimized by blowingair across the face of the carrier through the channels. The monolith isthen dried at 110° C. for one hour in an oven. The coated monolithcarrier is then cooled and is weighed in order to estimate the zincoxide loading Thereafter, the process of immersion in the slurry,draining-off of the free flowing excess, blowing air through thechannels and drying is repeated for such number of times as will resultin a coated monolith carrier having the desired coating thickness, i.e.,such that the layer(s) of zinc oxide will have a total thickness of atleast about 3 g/in³ of the carrier. Thereafter, the coated monolithcarrier is placed in a fused silica tray and calcined at 500° C. in airin a furnace for two hours.

[0020] The zinc oxide loading, in g/in³, on the monolith carrier isdetermined by using a weight-by-difference calculation:

Zno g/in³=final weight of calcined coated carrier−weight of uncoatedcarrier/ volume of monolith carrier

[0021] The following nonlimiting examples shall serve to illustrate theinvention. Unless otherwise indicated, all amounts and percentages areon a weight basis.

EXAMPLE

[0022] A cordierite cylindrical monolith carrier of 0.75 inches diameterand 1.5 inches length with 400 channels per square inch was placed in a1 inch diameter quart tube, using ceramic insulation to secure thesample in place. The carrier was placed in a reactor and brought to atemperature of 400° C. and a stream of nitrogen was then passed over thecarrier. Thereafter, the composition of the stream was changed to 40volume % hydrogen and 60 volume % nitrogen plus water vapor in an amountequal to 15 volume % of the stream. The signal from a hydrogensulfide-specific analyzer was then set as the zero value. Thereafter,hydrogen sulfide was introduced into the stream in an amount of 8 ppm byvolume. The hydrogen sulfide-enriched stream is passed through thesample and a measurement of the outlet gas concentration is periodicallytaken. When evidence of an increase of hydrogen sulfide in the outletgas is observed, the monolith carrier lifetime is recorded, and the gasstream is switched from the reactor to a bypass line and the inlethydrogen sulfide concentration was reduced to 1 ppm by volume. Thislatter step is used for instrument validation.

[0023] The results are shown in Table I in which a comparison is made ofthe zinc oxide layer thickness on the carrier measured in g/in³ of thecarrier versus the “breakthrough” time in hours. For the purpose ofthese examples, the “breakthrough” time is measured at the point thatthe concentration of hydrogen sulfide in the outlet gas streammeasured >50 ppb by volume. TABLE I Zinc Oxide Layer Thickness, g/in³Breakthrough Time, hours 2.1 1.75 2.6 2.0 3.7 8.1 3.9 8.4 5.1 9.0

[0024] The data set forth in Table I clearly show the dramatic increasein breakthrough time as the thickness of the zinc oxide coating on thecarrier is increased to a level above about 3.0 g/in³ of the carrier.

What is claimed is:
 1. In a process for the reduction of the level ofgaseous sulfur compounds in a gaseous stream wherein the stream iscontacted with a sorber capable of absorbing such compounds under sulfursorbing conditions, the improvement which comprises carrying out saidprocess with said sorber being present in the form of one or more layerson the surface of a monolith carrier, said layers having a totalthickness of at least about 3 g/in³ of the carrier.
 2. The process ofclaim 1, wherein at least three layers are present on the surface of themonolith carrier.
 3. The process of claim 1, wherein the total thicknessis at least 3.5 g/in³ of the carrier.
 4. The process of claim 1 whereinthe sorber comprises one or more metal compounds wherein the metal isselected from the group consisting of zinc, calcium, nickel, iron,copper and mixtures thereof.
 5. The process of claim 4 wherein the metalcomprises Zn.
 6. The process of claim 5 wherein the metal compoundcomprises zinc oxide.
 7. The process of claim 6 wherein the metalcompound comprises zinc titanate.
 8. The process of claim 1 wherein thegaseous sulfur compounds comprise H₂S.
 9. The process of claim 8 whereinthe initial concentration of H₂S in the gas stream prior to contact withthe sorber is about 0.15 to about 25 ppm and the stream is passed intocontact with the sorber at a volumetric hourly rate of about 500 toabout 100,000 volumes per volume of monolith carrier.
 10. The process ofclaim 9 wherein the initial concentration of H₂S in the gas stream priorto contact with the sorber is 0.25 to 10 ppm and the stream is passedinto contact with the sorber at a volumetric hourly rate of 2,500 to15,000 volumes per volume of carrier.
 11. The process of claim 7 whereinthe H₂S content of the gas stream after contact with the sorber isreduced to a level of less than about 100 ppb for a period of at least 1hour.
 12. The process of claim 11 wherein the H₂S content of the gasstream after contact with the sorber is reduced to a level of less than50 ppb for a period of at least 4 hours.
 13. The process of claim 1wherein the sorber is present on the surface of the monolith carrier inthe form of particles having an average particle size of 90%<10μ. 14.The process of claim 1 wherein the monolith carrier comprises a porousceramic.
 15. The process of claim 14 wherein the porous ceramic ispresent in the form of a foam.
 16. The process of claim 14 wherein theporous ceramic is selected from the group consisting of alumina,alumina-silica, alumina-silica-titania, mullite, cordierite, zirconia,zirconia-ceria, zirconia-spinel, zirconia-mullite and silicon-carbide.17. The process of claim 16 wherein the porous ceramic comprisescordierite.
 18. The process of claim 1 wherein the monolith carriercomprises a heat- and oxidation-resistant metal.
 19. The process ofclaim 18 wherein the metal is present in the form of a foam.
 20. Theprocess of claim 18 wherein the metal is selected from the groupconsisting of stainless steel and iron/chromium alloy.